Weighted NonSea Gem Production (non Rare)

[Edi]

What the chart also means is that there is a greater variety of sites available for those particular terrain types, but given that there are only 4 sites per province max, the total gem income for any given province is going to be more or less the same, all other things being equal. Which they are not, as the site frequency modifiers due to terrain kick in.

Swamps and wastes have a site frequency 10 percentage points greater than forests or mountains and 20 greater than plains.

The Looking for advice on site searching thread has some very relevant data for this analysis. It doesn't cross check terrains and paths, but it does break sites down by path and rarity and has the terrain modifiers.